This invention relates in general to systems for removing contaminants from liquids and, more specifically to a system for removing particles, both large or heavy particles and light, fine or buoyant particles from a liquid stream.
Water supplies for domestic drinking water, process water for chemical plants or other liquids are often contaminated with both large, heavy particles and fine, light, buoyant particles. These particles must be removed in a reliable, cost effective manner.
Many older water treatment plants use gravitational separation methods, typically in sedimentation systems or dual-media sand filtration systems that may not be acceptable under the newer water quality standards. In some cases, these systems can meet the standards through the use of properly mixed polymer chemical filter aids. The required expensive and complex polymer chemical mixing equipment requires constant attention, since the amount of the chemicals being added to raw water must be frequently readjusted to match the continually changing chemistry of the water being filtered. Slow sand filters require a considerable investment, but generally can be operate for longer periods without cleaning. Unfortunately, even with pretreatment, both dual-media and slow sand filters fail to meet water quality standards for hours or several days after each backwash cleaning. Contaminates have been found to pass through a sand filter whenever water flow rates are changed and whenever the pump is stopped and turned on again. In order to meet standards, it may be necessary to pump filtered water to waste after every backwash cycle, disposing of thousands of gallons of water, until the filter "ripens" or compacts. Thus, these filter systems are less than desirable for use today.
Particulate material has also been removed from liquids by floatation, another gravitational method, in which bubbles of a gas, such as air or oxygen, are introduced into the lower levels of the liquid and float to the top, carrying fine particles with them. Flotation is a gravitational method because the rise of bubbles is due to the gravitational acceleration acting on the mass of the liquid in accordance with the basic force equals mass time acceleration relationship. A force balance relative to a pocket of gas phase within liquid (a bubble), where the mass of the bubble is its volume times its density, shows that the bubble must rise to find equilibrium, because the density of a gas is generally less than that of a liquid.
Gravitational methods for separation of particles from a liquid, such as water, are particularly inefficient where the particles to be removed are close in density to water or so small that they remain suspended for very long periods of time. The requires a very large storage volume for a filter or floatation system to process large volumes of liquid.
Ozone, oxygen or air are sometimes mixed with water or other liquids to oxidize suspended solids, in particular organic contaminants. In conventional ozone contact chambers, the ozone gas is applied at the base of a tall column. The ozone-oxygen bubbles float to the surface slowly, their upward movement slowed by the downward counter flow of the water stream. To achieve sufficient contact time before the water passes from the mixing column, the column must be extremely tall and is difficult to install in ordinary sized plant equipment rooms. The concentration of dissolved ozone-oxygen is undesirably diluted in the larger vertical columns. While ozone-oxygen contact in mixing chambers is generally effective, there is a need for improved mixing in smaller mixing vessels.
Cyclone separators are often used to separate heavy particulate material from liquids such as water. Typical of such cyclone systems is that described by Laval in U.S. Pat. No. 3,568,837. In such separators, a fluid stream is directed at high velocity into a cylindrical tank at an angle that cause the fluid to rotate at high speed, driving entrained particles to the wall from which the particles move downward into a conical tank bottom with a central valve for removing the collected particles. The liquid is removed from the center top of the tank. However, conventional cyclone suspended solids separators only remove heavy particles, with any entrained gases and light or buoyant particles remaining in the exiting fluid stream.
A version of the cyclone type separator, called the hydrocyclone as originally described by Miller in U.S. Pat. No. 4,279,743 uses a cylindrical chamber having a mesh wall through which air is introduced into a rotating mixture of water and fine mineral particles. Bubbles are generated to which fine particles adhere. Bubbles and fine particles pass through an overflow outlet and water with heavy particles pass out through an underflow outlet. While effective in fine mineral separation, this device is not capable of separating both fine, light particles and dense, heavy particles from the liquid to clean the liquid. Further, since only inlet velocity cause the vortex to form and the very rough mesh surface exerts considerable drag on the rotating vortex, the mesh reduces vortex velocity and efficiency. Further, large amounts of heavy, large particles tend to clog the mesh surface.
Thus, there is a continuing need for a separation system that can rapidly and efficiently remove particles from liquids while treating a liquid, and can efficiently separate entrained gases and light or buoyant particles from the liquid in addition to heavy particles.